In a passive optical network, pulsed optical signals may be transmitted over a single optical fiber between one or more end units and a center servo unit. Each end unit may include a receiver and a transmitter able to operate in continuous mode, where continuous mode refers to transmission or reception of a continuous stream of pulses. The center servo unit may include a receiver and a transmitter using time division multiplexing to send and receive pulses in burst mode from each end unit, where burst mode refers to a group of pulses received within a specified time window.
The center servo unit may receive pulses in a nearly continuous stream comprising many bursts. However, pulses in a burst from one end unit may be transmitted with different amplitude than pulses in a burst from another end unit. There may be a substantial time gap between one burst and the next burst. A burst may include a pathological pattern, that is, a pattern having many pulses representative of one logic state and only a few pulses representative of another logic state. Some bursts representative of video signals, for example, may include pathological patterns.
Within each burst, there may be an initial time interval referred to as a “preamble” allocated to a sequence of training pulses and a subsequent time interval containing pulses representative of data. Pulses in a preamble are used by a receiver to set detection thresholds for distinguishing between pulses representative of a digital bit value, for example a bit value of “1”, from pulses representative of another digital bit value, for example a bit value of “0”, and to distinguish between adjacent bit values. It is preferred that a settling time for a receiver's response after the start of a new burst should be less than the time duration of the preamble portion of the burst. A receiver that takes longer to settle than the time duration of a burst's preamble may recover data incorrectly from the burst.
A group of pulses has an associated average DC (direct current) value related to an average of the individual amplitudes of the pulses within the group. The average DC value may be expressed as a voltage, a current, or as digital data. The average DC value may be used by a receiver to recover data from bursts transmitted through a fiber optic network. Because of differences between bursts as previously explained, a receiver may determine a new average DC value for each received burst.
Circuits referred to as integrators are used by some receivers for determining an average DC value for a group of pulses. In one configuration, an integrator is placed in a feedback path from an output to an input of a transconductance amplifier (TIA) to determine the average DC value for an input signal received by the TIA from a photodetector. This arrangement comprises a DC control loop operating to cancel the average DC value from the TIA output signal. A response time for an integrator may be related to a value referred to as an RC time constant, calculated as a product of value of resistance at an integrator input and a value of capacitance in a feedback path from an integrator input to an integrator output. The time for an integrator to determine an average DC value associated with a burst, referred to herein as a response time, is preferably less than the time duration of the preamble of the burst. The average DC value determined by an integrator preferably remains stable for the duration of the data portion of a burst to facilitate data recovery from the burst.
An integrator having an output response governed by a single RC time constant, referred to herein as an RC-based integrator, may be designed to output an accurate, stable DC signal corresponding to an average DC value of a signal comprising a continuous stream of equal-amplitude pulses. An RC-based integrator may prevent a DC offset present in an input to a TIA from contributing to the output response of the TIA for such an input signal. However, an RC-based integrator time constant value selected for determining an average DC value for a burst mode pulse signal may produce poor results. For example, an RC-based integrator having a low-frequency cutoff of 100 kHz, where the RC time constant for the integrator determines the cut-off frequency, could have a time constant of 1.59 μS. A DC control loop using an RC-based integrator with a time constant of 1.59 μS will settle within 7 μS a longer time duration than the duration of a “preamble” portion of a burst mode signal in conventional fiber optic communications systems. If, for example, the RC-based integrator in a receiver is to settle within a time duration of 350 nS, the corresponding RC-based integrator low-frequency cutoff would be 2 MHz, a relatively high value which may result in undesirably high jitter after the control loop settles. High jitter may lead to data recovery errors and other problems in fiber optic communications systems.
Designing an integrator operating with a single time constant for use with burst mode signals may lead to conflicting design requirements. The response time of the integrator is preferably within the preamble time of the burst, yet the integrator should preferably hold a DC output stable for the entire duration of the burst. Because the data portion of a burst signal may be much longer than the preamble portion, an integrator RC time constant selected to give a response time within the time duration of the preamble may not give an integrator output which is stable over the duration of the data portion of the burst, and may cause problems with for the TIA such as excessive jitter and sensitivity. Conversely, an RC time constant selected to maintain stable integrator output over the time duration of the data portion of a burst may be too large to permit an integrator to fully determine the correct average DC value during the preamble portion of the burst. Receivers having integrators operating with single time constants may therefore produce errors in digital data reconstructed from burst signals input to the receiver.